1. Field of the Invention
The invention relates to a light guiding film, and in particular relates to a light guiding film allowing light to enter and propagate therein.
2. Description of the Related Art
Mobile devices, such as mobile phones, PDAs, or digital cameras, have a side backlight module. Meanwhile, as smaller sizes are preferred, an extra-thin light guiding film is desired. When the light guiding film has a thickness less than 500 μm, it is difficult to direct light into the film from edges thereof. When the light source is a light emitting device (LED), light beams from the LED converge, and, therefore, can be directed into a thinner film than a cold cathode fluorescent lamp (CCFL) can. When the film is too thin, even light beams from LEDs cannot be directed into the film from edges thereof. A complicated method and system must be employed to couple light beams to the film.
A typical light guide film is provided in U.S. Pat. No. 6,259,854. Light beams are directed from edges of a thick light guide film and propagate by continuous total reflection therein. Light beams exits the light guiding film from a surface with a printed dot or micro optical structure. SID'03 DIGEST, 2003, p 1259-1261 discloses a light guiding film. Light beams are directed into the light guiding film from the back and then are reflected. The disclosed structure is complicated and reduces energy of light beams. US patent publication No. 2006/0262564 discloses an optical cap which has a crowned shape. Light beam from an LED is refracted into the light guiding film and propagates therein. The light beams are reflected and then exited from the film. Such a structure cannot have a small thickness and may increase costs. U.S. Pat. No. 5,883,684 discloses a side backlight module having a reflective layer reflecting light beams into a film. Here, light beams lose large amounts energy during refraction and reflection. US patent publication No. 2005/0259939 discloses a bended light guiding film, and light beams enter the film from the edge. US patent publication No. 2004/061440 discloses a thin light guiding film. Light beams are parallel to the film but do not propagate in the film. Japan patent publication No. 2000249837 discloses light beams being directed into a thin film by taper method. Such a method can reduce light intensity and requires longer optical couple distance. US patent publication No. 2004/202441 discloses a light guiding film with a notch corresponding to a light source. Light beams are directed into the film from edges. Applied Optics April 2006_Vol. 45 No. 12 and Optics Express 2008 March 2007/Vol. 15 No. 5 discloses grating diffracts light beams to propagate parallel to the film and couple the light beams to the film.